There are two ways to process back orders in Drupal Commerce:
autonomously with the help of the Back Order module, or
manually by the store administrator.
If you need to process back orders automatically, you will first need to install and enable the Back Order module. Once the module is enabled, you will need to set up the conditions under which back orders will be processed. This can be done by going to Configuration > Store > Back Order Settings (admin/commerce/config/backorder).
From the Back Order Settings page, you can specify the following:
Whether back orders are allowed
The Minimum Stock Threshold. This is the point at which the Back Order module will take action. If the stock level for a product falls below this threshold, the module will process a back order for the product.
The Maximum Stock Threshold. This is the point at which the Back Order module will stop processing back orders for a product. If the stock level for a product falls below this threshold, the module will not process a back order for the product.
The Order Status to assign to back orders.
If you need to process back orders manually, you can do so by going to the Orders page (admin/commerce/orders). From the Orders page, you can view all of the orders placed on your site. If an order needs to be processed as a back order, you can edit the order and change the order status to "Back Order".
